This article was published in #Issue 19 in summer 2016.

For some companies, concepts such as “lightweight, compact bass amps” or “neutral, transparent” tone are either newer concepts, or else they represent a smaller subset of said companies’ offerings. Fewer, still, feature products which are specifically designed to be equally appropriate for electric bass, acoustic bass guitar, and double bass. For Acoustic Image, though, these aren’t just concepts, and they don’t just apply them to a few products. These attributes are a part of the product DNA at Acoustic Image. Heck, they don’t stop at just making great products for all things “bass,” they make amplification products for just about any kind of instrument.

This unique approach breeds unique designs and products. Back in BGM #15, we introduced you to Rick Jones and Acoustic Image with our review of the Flex™ System (if you haven’t read it, check it out for some added background on Rick and AI). While working on that review, I reflected back on my experience with the first “micro” head that really grabbed my attention, the original AI Clarus SL™. These fond memories, and my realization of the technical advances evident in the Flex System, made me really interested to try out the newly reintroduced “Series 4plus” Clarus SL.

Smaller, Lighter … More Powerful … Cheaper?

As with the original, the “SL” stands for “Smaller Lighter.” The original Clarus SL put out 250 watts into a 4-ohm load (it could hit 350 watts into a 2-ohm load). The latest iteration is approximately twice as powerful (AI claims 500 watts “rms continuous,” and 650 watts “rms music”), and is still designed to handle a 2-ohm load. In addition to upping the ante on the output section side of the equation, AI also enhanced the front end, with an updated tone stack, a variable high-pass filter, and additional front panel controls. These are impressive advancements, and AI should be very pleased with their ability to hit these targeted improvements over the original SL. Of course, with all of these advancements, and double the power, of course, the S4 version of the Clarus SL costs mo … wait a minute … it costs less? Twice the power, more features, smaller form factor, AND it costs less? Man, Rick seems like a really smart guy, but I may need to speak to him about his product pricing strategy. What’s that? You don’t want me to tell Rick that AI is not charging enough for this gear? Alright, then…

Details and Features

The Clarus SL features two ¼” inputs, labelled Hi Z and Lo Z, both of which can be used at the same time. The Lo Z input features a 1 Mohm input impedance, which works great for electric basses with magnetic pickups. The Hi Z is rated for 10 Mohm, which is ideal for piezo-equipped instruments, like acoustic bass guitar and most double bass pickup systems. Following the inputs, we find the input gain Level control. Like the Flex Pre, the Clarus SL employs an excellent 4-band EQ. The Low control is a shelving-type, affecting frequencies below 30Hz. The Lo Mid control affects frequencies between 70Hz and 700Hz (centered at 250Hz). The Hi Mid control affects frequencies between 700Hz and 3kHz (centered at 1.2kHz). The Hi control is a shelving-type, centered at 10kHz. The first three bands offer 12 dB of boost or cut, whereas the High control is +/- 15 dB. The SL also includes a single-knob iteration of their variable high-pass filter, labeled “Filter.” This knob allows you to vary the starting point of the 12 dB/octave rolloff between 30Hz and 180Hz.

Like the Flex Pre, the Master volume control comes right after the balanced XLR Direct Out (which conveys the combined output of both channels), so the input Level control affects the level of the DI signal, but the Master volume does not. The remaining front-panel controls include push-button switches for Post EQ (the signal is pre-EQ if the Post EQ switch is not engaged), Mute, DI Pad (10 dB), and Ground Lift.

The back panel is fairly sparse, as far as features are concerned. Pretty much all you’ll find is the power switch, IEC socket (with user-accessible fuse), and two Speakon® (non-combo) speaker output connectors. However, there’s lots to read, and good information to be learned. For instance, the power supply has an automatic voltage switch, and can handle inputs anywhere from 100V to 240V. AI also proudly displays it 2Ω min rating, just above the speaker outputs. While I do like the ¼” & Speakon combo jacks, AI does include a Speakon to ¼” adapter, so if you have been using ¼” speaker cables, you are still good to go (but I would recommend switching to the more robust Speakon connections).

On the whole, the Clarus SL does a great job of giving you all the features and controls which you might actually use, and doesn’t clutter things up with features you don’t really need. Notably, it does not have (or need) a fan, so fan noise is never an issue. About the only thing I can think of to raise as a potential negative is that if you are using a shorter bass cab (or shorter stack of cabs), and you are looking down on the SL as you tweak the controls, it can be hard to read the labels for the controls, as they are blocked by the knobs. Fortunately, they are laid out very intuitively, and of course, if you are looking at the head more straight-on, they are easy to read.

What Were They Thinking?

AI’s CEO, Rick Jones, is not only really smart, he’s also a pretty approachable kinda guy. So I approached Rick with some questions about the Clarus SL.

TB – The Clarus SL is an impressive blend of features, power and high-fidelity sound, in a box that is decidedly small, but yet not cramped. Talk me through some of the design goals for the SL, relative to its larger “brothers” in the Clarus line.

RJ – We had discontinued the one-channel version of the Clarus and needed a one-channel amp to complete our product line. One of our customers suggested that we bring back the Clarus SL, which we introduced in April of 2004, and discontinued a couple of years later. I thought that was a good idea. When first introduced, the SL was at the top of our product line. Its compact size and early use of a switched-mode power supply meant it was expensive. So, in reintroducing it, I wanted to bring it back as a more basic model at a lower cost; the entry level of the product line, rather than the top-of-the-line. At the same time, I wanted to incorporate as many of the features of the Clarus amp as was possible, considering the cost goals and size.

So, we took the design of our two-channel preamp (Flex) and turned it into a single-channel design that retained the four-band EQ, the variable-frequency high-pass filter, the mute, pre/post DI, pad and ground lift features (which the original SL didn’t have) and put it in to a smaller form factor. For the SL-R model, we left space for the digital effects, which are the same as in the Flex preamp. We also used the same power amp module as is in the Clarus to give it the same power capability as the Clarus.

We were able to fit it all into a chassis that is slightly smaller than the original SL. I’ve attached a photo to show the comparison. And, the street price came in at just about half of the original SL, so we were happy about that.


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TB – How were you able to address the thermal/cooling issues without resorting to the use of a fan? And this is on a head rated down to 2 ohms, no less!

RJ – I don’t like fans, so I looked for passive means to provide heat sinking for the power amp module. In the Clarus design, we came up with a very effective passive heat sink, which we have been using for several years, now. We applied the same approach to the SL, shrinking the size a bit in the process.

TB – As with the rest of your Clarus amp line, the SL is a full-range amp that is appropriate for a wide range of instruments. Do you have any idea what percentage of SL buyers are using them for double bass, electric bass, jazz guitar, or other instruments?

RJ – As you pointed out, the amp is designed to be used for all instruments, but I’m not aware of the breakdown for different instruments. At this point in time, the SL is primarily used by acoustic and electric bassists, the SL-R by guitarists. Once the word gets out, I expect other players will be interested in it.

How Does It Sound?

Much like my experience with the Flex System, the Clarus SL impresses with its very neutral, transparent tone. While the tone is decidedly “smooth,” this does not come at the expense of clarity or high frequency extension. The midrange is solid and balanced throughout its range. The Filter control really lets you dial in the low end. It is a deceptively simple and incredibly useful single-knob tool. As with the Flex System, as I played different instruments through the SL, each instrument had its own voice, and I always felt like I was hearing more “instrument” than “rig.”

With some of my bands, I do a lot of doubling (switching between electric bass guitar and double bass), and sometimes I am also throwing acoustic bass guitar into the mix. These instruments present very different rig challenges. An amp/cab combination that might work great for a P-bass might sound “fake” with a piezo-equipped double bass. Rigs that excel on double bass sometimes lack “slam” and “punch” on electric bass. While much depends on which bass enclosure(s) you bring to the gig, the Clarus SL is definitely up to the task of faithfully – and “naturally” – reproducing whatever instrument you bring to the gig. The two inputs (Hi Z and Lo Z) offer great flexibility, and a quick tweak of the Filter knob really opens up the possibilities. Granted, it is a very clean-sounding head, so if you want some dirt, you may need to supply it with a pedal (or two), but the SL is quite pedal-friendly.

Since the Clarus SL is a head – and the Flex System was an integrated preamp/power cab setup – I was able to try the SL through a variety of different bass enclosures. Most of them were the kind of full-range cab that helped show off the sonic purity of the AI head, but some of them were definitely rather “voiced.” I am happy to say that the Clarus SL yielded pleasing and predictable results no matter what cab or cabs I hooked up. It is small enough to feel at home with a compact 1x10 or 1x12, but it is more than capable of driving an 8x10. Heck, it can push two 8x10’s!

It does take a little while to wrap your head around the amount of real-world output power the little SL has on hand. First off, its diminutive size and weight encourage your brain to categorize the Clarus SL as a “small” amp, likely with modest output power. It is about the same size as my GK MB200, which I really like, but which is limited to 200 watts (into 4 ohms). The SL is capable of putting out more than 500 watts into this same load. That is no small feat. What’s even more impressive is that the little SL is stable down to 2 ohms, where it puts out around 600 watts! And it does all this without a fan. Mind-blowing…

The Level and Master pots both feel like they have a very linear taper, and you can definitely use the whole range of the knob. On some amps, you’ve got about all you are going to get (without clipping) by the time you get the gain/volume knobs to noon or maybe 1 o’clock. No so with the Clarus. You do need to be willing to crank things up a bit. I routinely ran the Level and Master controls up well past 2 o’clock. The power of the SL really shines when you drive it to 4 ohms and beyond. I really loved it with three 1x12’s (2.67-ohm load), where it packed punch and slam worthy of any hard rock gig (but of course, all the while maintaining a very clear, neutral, balanced tone). Again, you have to be willing to crank things up to get all the volume capabilities out of the SL, but when you do crank it, it absolutely delivers.

Much like the Flex, the Clarus SL is almost totally lacking in noise or hiss. Crank the Master, hold the strings, and you can hardly tell it’s on. What’s more, with the Mute engaged, it is dead silent. I mean silent. No hiss. Nothing! That is rare, in my experience. Most heads exhibit some level of hiss or noise when turned on, but muted. Once again, AI impresses with the attention to details.

The Bottom Line

The Clarus SL may be the most exciting, but unassuming, head I have encountered. It has a classy, understated vibe about it, but when you put it through its paces, it is just so undeniably competent. The feature set offers everything you need, but nothing you don’t. The 4-band EQ is excellent, and the variable high-pass Filter is one of the best one-knob tone controls out there. Sonically excellent, the Clarus SL also packs considerable power, but stays silent when you want it to. This thing should be bigger than it is, and it should cost more than it does. Oh, and you’d think it would need a fan. But it doesn’t. And it’s tiny. And the price is crazy reasonable. This is a great head, folks. You should definitely check one out!